Ad-hoc networks are getting attention as a scheme of establishing a flexible network by a mobile terminal at the location thereof. Ad-hoc networks are based on an assumption that a network is closed between terminals through a close-range communication as much as possible during a communication. Example close-range communications are ZigBee, Bluetooth (registered trademark), UWB (Ultra Wide Band), and RFID (Radio Frequency Identification). Those are classified in a wireless PAN (Personal Area Network) specification in a point that a communication range is relatively short although there are differences from each other in a communication speed and a power consumption.
When an ad-hoc network is used, a terminal possessed by, for example, a child, an elder person, or a disability person transmits personal information to the effect that such a person has a disadvantage in the age or a leg to terminals buried around a town, a barrier-free environment can be realized. A barrier-free environment means an environment such that the stages of store shelves are ascended or descended as needed, and a product is disposed so that a person can easily reach. Information exchanged at this time contains a large amount of privacy information. Hence, it is necessary to safely protect a communication through an information security like an encryption.
It is not always true that a manager (server) responsible for a network management including a security securement with each mobile terminal is capable of communication on a steady basis. That is, an opportunity at which the manager is capable of communication is sometimes limited to an opportunity at which a certain infrastructure is available such that a hot spot is available. Hence, in order to realize a safe communication through an ad-hoc network, it is necessary for each mobile terminal to have necessary security setting done in advance in an opportunity at which such a mobile terminal is capable of communicating with the server. A simplest scheme of realizing such a security setting is to cause respective mobile terminals to have one common key shared over the network. According to this scheme, however, when the common key is leaked from just one terminal in the network, the communication becomes vulnerable.
With respect to such a disadvantage, Patent Literature 1 discloses a method of causing a pair of communicating terminals to store a key in advance, thereby accomplishing a safe communication. Moreover, Patent Literature 2 discloses a method of causing a terminal and another terminal to share a key at a high probability under a condition in which another terminal is surely present near the former terminal.